1. Field of the Invention
The field of the invention is electrical contact apparatus for energizing and deenergizing a superconductive circuit and, in particular, such apparatus having leads which are automatically extended and retracted to make and break contacts with the circuit at a cryogenic temperature.
2. Discussion
Magnetic resonance (MR) magnets are well known for diagnostic imaging purposes. These magnets typically include a superconductive magnet coil assembly which is immersed in liquid helium to reduce the temperature of the coil assembly to being at or below the critical temperature at which the coil assembly enters the superconductive state. In the superconductive state, the coil has an electrical resistance of zero ohms so that it can conduct a very large "persistent" current. The current generates a correspondingly high magnetic field which is used for the diagnostic imaging purposes.
Such MR magnets may be stationed in a special room therefor or may be part of a mobile system in which the magnet is transported in a truck or van to different locations where it is used. Magnets which produce less intense magnetic fields (e.g. less than 0.5 Tesla) may be constructed so as to substantially contain the magnetic field in the volume of interest by using magnetic shields. However, a shield for a more powerful magnet (e.g. 1.5 Tesla) would be too heavy to transport and therefore it is not practical to shield such a magnet. Rather, the more powerful magnets are deenergized prior to transport, transported in the deenergized state, and then reenergized or ramped at the location of use. For less powerful magnets and magnets which are installed in buildings, energizing and de-energizing the magnet is infrequent and may only occur once in the life of the magnet.
For the powerful, transportable magnets, recharging and discharging may occur two or more times per day depending upon the number of locations to be serviced on any given day. Thus, it is often necessary on the powerful, portable magnets to make and break connections with the magnet coil to energize and de-energize the magnet. The connections are usually made and broken deep inside the cryostat at the cryogenic temperature. A penetration tube of the cryostat provides access to the terminals on the superconducting magnet. Such penetration tubes are discussed in patents such as U.S. Pat. No. 4,526,015. Such penetration tubes generally penetrate the cryostat through one or more vacuum or other insulating layers, and into the helium vessel in which the magnet coils and liquid helium are contained at or somewhat above atmospheric pressure. Such penetration tubes may be either vertical, horizontal or could be at some other orientation. Making and breaking the connections at the cryogenic temperature minimizes vaporization of the liquid helium inside the cryostat because after the magnet is energized, the leads can be retracted out of thermally conductive contact with the superconducting circuit and out of the space which is at or near the cryogenic temperature to minimize heat transfer through the leads to the cold space.
For most MR magnets, the connections with the main magnet coil for energizing and de-energizing the magnet have been made manually. However, this is cumbersome for the powerful, portable magnets for which such connections may be made quite often. Also, in the van or truck in which such magnets are transported, the space around the cryostat for access to the penetration tube can be very limited, which hampers manual connection.
The penetration tube contains helium vapor and it is desirable to seal the top of the penetration tube to a pressure slightly above atmospheric (e.g., 1.5 psig) to conserve helium. A recondenser provides refrigeration of the cold space to make up for heat losses and maintain the pressure in the operating range. In the invention, a sliding seal is provided between the leads and the top of the penetration tube. However, because it is necessary to cool the leads by circulating helium through them when they are energizing the magnet, and also because the leads are made of metal, which is a good thermal conductor, it has been found that, when energizing the magnet coil, the leads have a tendency to freeze up and become bound to the top of the penetration tube such that they cannot be retracted.